1. Field of the Invention
The present invention relates to a method and an apparatus for recording halftone images, and more specifically to a technique to effectively prevent rosette moire.
2. Description of the Related Art
In prepress process of color printing, an original color image is separated into four color separation images of four primary colors, and a set of the color separation images are recorded on photosensitive films as halftone images. Color printing plates for the respective colors are then produced from the set of halftone image films. The halftone images of the primary colors are printed on a printing sheet one after another to make a color print.
If all of the four halftone images which represent a color image have the same halftone dot structure, registering mismatch between some of the halftone images on a printing sheet often causes undesirable visual color shift in a resulting printed matter. In order to prevent the undesirable color shift, a set of halftone images are generally produced to have different screen angles, for example, 0, 15, 45, and 75 degrees for four primary colors, respectively. The screen angle is an angle between a scanning line and a grid line drawn through halftone dot centers.
If a set of halftone images which have different screen angles are printed on a printing sheet, however, the difference in the screen angle often causes circular patterns, or "rosette moire", appearing at regular intervals. FIG. 18 shows an example of "rosette moire". Halftone dots of three primary colors of magenta (M), cyan (C), and black (K) are represented by solid squares, open circles, and solid circles, respectively. As shown in FIG. 18, a circular pattern consisting of plural halftone dots of the three colors is formed around a meeting point MP of the three halftone dots of respective colors. With naked eyes, many circular patterns having a pitch Prm of the meeting points MP are observed on a printed image. Generally, the pitch Prm of the rosette moire is greater than a pitch of halftone dots; in FIG. 18, Prm is approximately 2.2 times a pitch Pc of cyan halftone dots. The greater the pitch Prm of rosette moire, the more conspicuous the rosette moire becomes to naked eyes. The rosette moire is often observed over a wide range on a printing image, thereby deteriorating the quality of the printing image.